Scanned-proximity Probe Microscopes (SPM's) are instruments that have been in use in universities and industrial research laboratories since the early 1980's. These instruments allow for various imaging of surfaces as well as measurement of the intermolecular forces between two surfaces (or a small tip and a flat surface) in vapors or liquids with a distance resolution of 1 Å. This means that images and forces can be obtained at the atomic level. Over the years, the technique has been improved and its scope extended so that it is now capable of measuring many different surface properties and phenomena.
One type of SPM is an atomic force microscope (AFM), which generally consists of a sample surface and a probe that is supported at the end of a force-measuring cantilever spring. The AFM works measures a local property such as height, optical absorption, or magnetism, with a probe or “tip” placed very close to a sample. It operates by first positioning the tip near the surface and then moving the tip or surface vertically (contact or tapping mode) or laterally (scanning mode) while measuring the force produced on the tip by the surface. The force is calculated by measuring the deflection of the cantilever spring supporting the tip.
The most common method of measuring deflection in an AFM is the optical or beam deflection method where vertical deflection can generally be measured with picometer resolution. The method works by reflecting a laser beam off end of the cantilever. Angular deflection of the cantilever causes a twofold larger angular deflection of the laser beam. The reflected beam strikes a split photodiode (i.e., two side-by-side photodiodes) and the difference between the two photodiode signals indicates the position of the laser beam on the split photodiode and thus the angular deflection of the cantilever.
FIGS. 23a and b illustrate a typical setup of an AFM. The laser 1000 outputs a laser beam 1002 that is pointed at the cantilever 1004. A piezoelectric scanner 1006 is used to position the sample. The laser beam 1002 deflects off the cantilever 1004 and is reflected into the split photo-diode 1008 via mirror 1010. The output of the split photo-diode is conditioned via module 1012 and is input into feedback control module 1014 that is used to control the position of the sample movement of piezoelectric scanner 1006. In static force spectroscopy the cantilever deflection is solely due to the tip-sample inter-action. The piezoelectric scanner 1006 is rastered in the lateral directions and the deflection of the tip is used to interpret sample properties. In the dynamic mode, the cantilever support 1016 is forced sinusoidally using a dither piezo 1018. The changes in the oscillations caused by the sample are interpreted to obtain its properties.
The cantilever has low stiffness and high resonant frequency that allows it to probe interatomic forces. Micro-cantilevers, which are cantilevers having lengths typically ranging from 100 to 200 μm with tips of 5 nm, have been utilized in biological sciences to perform feats such as cutting DNA strands and monitoring RNA activity. Another application of the micro-cantilever is in the detection of single electron spin that has significant ramifications for quantum computing technology.
In spite of the underlying promise, considerable challenges remain. Pivotal to harnessing the vast potential of micro-cantilever based technology is ultra-fast interrogation capabilities. This is apparent as the manipulation, interrogation and control of atoms or spins of electrons needs to be accomplished for material that has macroscopic dimensions. To achieve high throughput, fast interrogation is imperative. It is becoming increasingly evident that for many nanotechnological studies, high bandwidth is a necessity. For example, in the field of cell biology, proposals on using nanotechnology have been presented where nano-probes track events in the cell. These events often have time-scales in the micro-second or nano-second regimes. Current measurement techniques do not meet the aforementioned high precision and bandwidth requirements. For example, the micro-cantilever is often operated in the dynamic mode where it is forced sinusoidally by a dither-piezo. This mode of operation has advantages of better signal to noise ratio and being gentle on the sample. Most dynamic imaging methods employing micro-cantilevers currently use variables such as the amplitude and phase, or the equivalent frequency of the micro-cantilever to infer sample characteristics. These are steady state characteristics and do not hold much significance during the transient of the cantilever oscillation. The present methods are therefore inherently slow owing to the large settling times of the cantilever oscillations.